1. Field of the Invention
The present invention is generally related to a method and apparatus for performing quantitative analysis of certain chemical species in the gaseous, liquid, or solid state. More specifically, the present invention is directed to a sensor which utilizes an optical fiber interferometer to detect the absorption of a modulated laser beam by a particular chemical species. The present invention is also directed to a means and method to monitor optical absorption in an optical fiber.
2. Description of the Prior Art
Fiber optics have, in many ways, supplanted metal conductors as the favored information transfer medium choice for both science and industry. In this connection, fiber optics have been widely employed in a variety of telecommunication applications. Fiber optics have also been utilized in sensitive sensor systems such as those disclosed in Applicant's U.S. copending application Ser. No. 286,058, which is herein incorporated by reference.
Fiber optic sensor systems are attractive for a number of reasons including immunity to electromagnetic interference, ability to operate in a wide variety of environments, high sensitivity, and the potential for multiplexing. Chemical sensors incorporating fiber optics have generally utilized "optrodes" or materials which undergo changes in optical properties when interacting with a chemical of interest. The optrode is usually deposited on the end of the fiber. When chemically acted upon, the chemical reaction affects the fluorescence properties of the optrode as well as its reflectance or transmittance.
The use of optrodes, however, has not gained widespread practical application. This is believed to be due in part to the relatively slow response times of optrodes which is usually measured in term of minutes. Furthermore, optrodes generally show relatively low sensitivity and nonlinear response characteristics which change over extended time periods of use. Additionally, each optrode generally can only be used to detect one chemical species.